1. Field of the Invention
This invention relates to novel dielectric ceramic compositions which are adapted for use in microwave ranges. The invention also relates to microwave devices using the above compositions.
2. Description of the Prior Art
In recent year, mobile communication devices which make use of electromagnetic waves in the microwave range have been conspicuously developed. Typical of such devices are car telephones, portable telephones and the like. This, in turn, very strongly demands the miniaturization of the devices. In order to meet the demand, it will be necessary to miniaturize individual components of the devices. Dielectric ceramics have been frequently built in, as a dielectric resonator, resonator components of these devices such as filter elements, oscillator elements and the like. For the same resonance mode, the size of the dielectric resonator is inversely proportional to the square root of the dielectric constant of the dielectric ceramic used. In order to make a smaller-size dielectric resonator, use of a dielectric ceramic having a higher dielectric constant is essential. Other characteristic properties which the dielectric ceramics should have include a low loss in the microwave range, i.e. a high value of unloaded Q, and a small temperature variation of resonance frequency, i.e. a small temperature variation of dielectric constant.
Up to now, a great number of ceramics have been proposed for use as dielectric resonators. For instance, BaO-TiO.sub.2 -Sm.sub.2 O.sub.3 ceramics which have been disclosed in U.S. Pat. No. 4,330,631 are set forth as having a high dielectric constant. More particularly, the ceramics have a specific dielectric constant of approximately 80, an unloaded Q value as high as approximately 3000 at 2 to 4 GHz, and a small temperature coefficient of resonance frequency. As ceramics whose dielectric constant is over 90, there have been reported BaO-PbO-TiO.sub.2 -Nd.sub.2 O.sub.3 ceramics in Journal of American Ceramic Society Vol. 67(1984), pp. 278-281.
For realizing miniaturization and high performance of resonator components, there is known a multilayer structure of conductor layers and ceramic layers. The conductors used for the multilayer device should have high conductivity because of the use in a high frequency range and have to be made of Cu, Ag, Au or alloys thereof. On the other hand, the dielectric ceramics for such devices should be sintered along with the conductor metals. More particularly, the ceramics should be sintered under conditions where the conductor metals are not molten and oxidized. To this end, the ceramic material should be densely sintered at low temperatures not higher than 1075.degree. C. If Cu is used as the conductor, the ceramic material does not suffer characteristic degradation when sintered at a low partial pressure of oxygen.
Known ceramics for microwave service including the afore-described dielectric ceramics have a sintering temperature as high as approximately 1300.degree. C. Eventually, these ceramics cannot be sintered simultaneously with the conductor metals having high electric conductivity, making it difficult to provide a multilayer device.
Dielectric ceramics which are sinterable at low temperatures include Bi-based ceramics. Among them, Bi.sub.2 (ZnNb.sub.2)O.sub.6 -Bi.sub.3 (Ni.sub.2 Nb).sub.9 ceramics which are disclosed in U.S. Pat. No. 4,638,401 are set forth as adapted for a multilayer capacitor. The ceramics are sinterable at approximately 950.degree. C. and have a high dielectric constant of 90 and a good temperature characteristic of the dielectric constant.
However, there have never been known any dielectric ceramics which can be used in the microwave range, have a high dielectric constant and are sintered at low temperatures. The above-mentioned Bi-based ceramics are of no exception for this purpose.